The present invention relates to an improved coaxial cable for radio frequency signal coupling.
An open type coaxial cable of the prior art is shown in the side view and sectional view of FIGS. 1 (a) and (b), respectively. The cable comprises coaxially arranged inner and outer conductors 1 and 2, respectively. A continuous slot 3 is provided in the outer conductor along the length of the cable. This type of cable makes use of the outwardly protruding portion of the TEM mode electromagnetic field which propagates in the space between the inner and outer conductors for purposes of communicating with an object adjacent the cable. Such object may be a train, an automobile, an industrial crane, or the like, may be movable or stationary, and would typically mount a signal pickup device on the side of the object facing the coaxial cable slot. The communication between the coaxial cable and moving object may be to implement remote control, or simply to convey information. The degree of radio-wave coupling between the cable and such a moving object travelling therealong often cannot be made sufficiently large, because the coaxial cable shown FIG. 1 has no mechanism for positively radiating leakage waves from the slot. Further, the protruding external electromagnetic field coupling the cable and the moving object may adversely affect the signal transmission of the cable due to the influence of the outer environment surrounding the cable, e.g., pollution, water, etc. Consequently, the quality of signal transmission may become fatally deteriorated. The greater the amount of radio-wave coupling required to be taken out, the more the width of the slot must be increased. This widening accentuates the effects of outer environmental changes, whereby stabilized radio-wave coupling and transmission is increasingly degraded.